Process for producing a catalyst for denitration by catalytic reduction using ammonia

ABSTRACT

A catalyst for denitration by catalytic reduction using ammonia capable of preventing catalyst deterioation due to vapors of heavy metal compounds contained in exhaust gases and having a high strength and a superior resistance to poisons and a process for producing the catalyst are provided, which catalyst comprises TiO 2 , oxide(s) of at least one of V, Cu, Fe and Mn and oxide(s) of at least one of Mo, W and Sn, the total of the mol number(s) of the oxide(s) of at least one of Mo, W and Sn falling within a range of 2×10 -6  to 20×10 -6  mol/m 2  per the unit specific surface of the catalyst, and which process comprises having oxide(s) of at least one of Mo, W and Sn adsorbed and supported onto a composition comprising TiO 2  prepared in advance and oxide(s) of at least one of V, Fe, Cu and Mn so as to give the above-specified total of the mol number(s) of oxide(s) of at least one of Mo, W and Sn.

This is a continuation of copending Ser. No. 201,536 filed June 2, 1988,now U.S. Pat. No. 4,966,882.

BACK GROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a catalyst for denitration by catalyticreduction using ammonia and a process for producing the catalyst. Moreparticularly, it relates to a denitration catalyst suitable fordenitrating exhaust gases containing a large quantity of heavy metaloxides.

2. Description of the Related Art

Nitrogen oxides (NOx) exhausted from various fixed NOx - generatingsources are a main air pollutant together with sulfur oxides (SOx) As tothe process for removing this NOx, there are various processes, andamong these, a process of selectively reducing NOx with ammonia added toexhaust gases in the presence of a catalyst has been constituting themain current of the processes. It is required for this catalyst fordenitration by catalytic reduction using ammonia to be not degeneratedby SOx and ashes contained in exhaust gases generated by combustion offossil fuel such as petroleum, coal, etc., and for satisfying thisrequirement, various catalysts based on titanium oxide have beeninvented and now broadly practically used (Japanese patent applicationliquid-open Nos. Sho 50-128681/1975, Sho 53-28148/1978, etc.).

These catalysts are prepared by adding oxides of transition metalelements such as vanadium, molybdenum, tungsten, iron, chromium, etc. tometatitanic acid or titanium oxide by means of kneading, impregnation orthe like, followed by calcination, and have superior activity and lifeto usual catalysts for denitration of exhaust gases from a combustionapparatus of petroleum, coal, etc.

However, in the case where exhaust gases containing a large quantity ofvapors of heavy metal oxides such as combustion exhaust gases from lowquality coal, exhaust gases from boilers having an ash-circulating line,etc. are treated, a problem of reduction in the catalyst activity hasnot been taken into consideration. In particular, in the case of acombustion system having an ash-circulating line 9 as shown in FIG. 5,metals contained in mineral substances of coal such as lead (Pb),selenium (Se), arsenic (As), cadmium (Cd), zinc (Zn), etc. aretransferred into exhaust gases in the form of vapor of single substancesor oxides during the process wherein ashes are molten in a furnace 1 andrecovered in the form of slug, and are present in the form of a highconcentration of metal vapors upstream of an air preheater 6 usuallyprovided with a denitration means (see H. Brumsack et al., EnvironmetalTechnology Letters; 5, 7-22 (1984)), and it has raised a problem thatthe denitration catalyst is poisoned by these vapors (see Shokubai, 29,36-37 (1987)).

Further, the above-mentioned prior art has given no adequateconsideration to the catalyst strength, particularly to the hardwearingstrength of the catalyst due to ash particles contained in exhaustgases.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a catalyst whichis prevented from its deterioration due to vapors of heavy metalcompounds contained in exhaust gases.

A second object of the present invention is to provide a process forproducing a catalyst having overcome the drawbacks of theabove-mentioned prior art and having a high strength and a superiorresistance to poisons with a simple process.

The above first object is achieved by having an oxide or oxides of atleast one element selected from among molybdenum (Mo), tungsten (W) andtin (Sn) adsorbed and supported onto a composition composed mainly oftitanium oxide and an oxide or oxides of at least one element selectedfrom among vanadium (V), copper (Cu), iron (Fe) and manganese (Mn) inspecified proportions.

The above specified proportions of the metal oxides are achieved byimparing the catalyst so that the content of Mo, W or Sn per thespecific surface area of the catalyst at the state where the catalyst isused can satisfy the following relationship:

    2×10.sup.-6 ≦M/SA≦20×10.sup.-6

wherein M represents the number of mol (mol/g) of Mo, W or Sn per unitweight of catalyst and SA represents the specific surface area ofcatalyst (m² /g).

Namely, the present invention resides in;

a catalyst for denitration comprising titanium oxide, an oxide or oxidesof at least one element selected from the group consisting of vanadium,copper, iron and manganese and an oxide or oxides of at least oneelement selected from the group consisting of molybdenum, tungsten andtin, the total of the mol number(s) of said oxide or oxides of at leastone element selected from the group consisting of molybdenum, tungstenand tin falling within a range of 2×10⁻⁶ to 20×10⁻⁶ mol/m² per thespecific surface area of said catalyst.

Further, the present invention resides in;

a process for producing a catalyst for denitration which compriseshaving an oxide or oxides of at least one element selected from thegroup consisting of molybdenum, tungsten and tin adsorbed and supportedonto a composition comprising titanium oxide prepared in advance and anoxide or oxides of at least one element selected from the groupconsisting of vanadium, iron, copper and manganese so that the total ofthe mol number(s) of said oxide or oxides of at least one elementselected from the group consisting of molybdenum, tungsten and tin canfall within a range of 2×10⁻⁶ to 20×10⁻⁶ mol/m² per the specific surfacearea of said catalyst.

The above-mentioned second object of the present invention is achievedby;

a process for producing a catalyst for denitration which comprisesadding a compound or compounds of at least one element selected from thegroup consisting of vanadium (V), iron (Fe), copper (Cu) and manganese(Mn) to titanic acid, kneading and drying the mixture, threaftercalcining it at a temperature of 300° to 600° C., adding molybdenumtrioxide (MoO3) to the calcined material, wet-kneading it, molding thekneaded material into a definite shape, drying the molded material andcalcining it at a temperature of 400° to 600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a chart illustrating the relationship between the change inquantity of MoO₃ per specific surface area and percentage denitration inExamples 2-6 of the present invention.

FIG. 2 shows a chart illustrating relationship between the change in thepreliminary calcination temperature and percentage denitration inExample 16 of the present invention.

FIG. 3 shows a chart illustrating the relationship between the change inthe normal calcination temperature and the percentage denitration inExample 16.

FIG. 4 shows a chart illustrating the X-ray diffraction pattern of thecatalyst of Example 16.

FIG. 5 shows a flow sheet of combustion system of coal provided with anash-circulating line.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is considered that conventional titanium catalysts are activated byadsorption of sulfate group onto the catalysts and when vapors of heavymetal compounds are contacted therewith, activity reduction occurs byreplacement of the sulfate group by the compounds.

Whereas, in the case of the catalyst of the present invention, it isconsidered that the oxide(s) of at least one of Mo, W and Sn on thecatalyst play a role of activating the catalyst in place of sulfategroup, and even when the oxide(s) are contacted with vapors of heavymetal compounds, the adsorbed molecules of the oxides) of at least oneof Mo, W and Sn are not replaced by vapors of heavy metal compoundsunlike sulfate group; hence no activity reduction occurs.

The catalyst of the present invention is produced basically by havingthe oxide(s) of at least one of Mo, W and Sn adsorbed and supported ontoa catalyst composed of titanium oxide and oxide(s) of at least oneelement selected from among V, Fe, Mn and Cu, concretely, the catalystis produced by adding compound(s) of at least one of V, Cu Fe and Mn totitanic acid or titanium oxide in a conventional manner such askneading, impregnation or the like, followed by making up the resultingmaterial into a catalyst through steps such as drying, calcination,molding, etc., and then contacting the catalyst with a gas containingvapor(s) of the oxide(s) of at least one of molybdenum trioxide (MoO₃),molybdenic acid (H₂ MoO₄), tungsten trioxide (WO₃) and tin oxide tothereby have the metal oxide(s) adsorbed thereonto.

The atomic ratio of at least one of vanadium, copper, iron and manganeseto at least one of molybdenum, tungsten and tin in the catalyst of thepresent invention is preferably in the range of 1/10 to 4/10.

The above-mentioned adsorption process of the oxide(s) of at least oneof Mo, W and Sn by using the vapor(s) of these metal may be replaced bycontacting the above-mentioned catalyst with vapor(s) of alkylcompound(s), alkoxide(s) or the like containing at least one of thesemetals and forming the oxide(s) thereof during the preparation of thecatalyst. Further, a process may comprise a step of adding thecompound(s) of at least one of Mo, W and Sn to the catalyst.

At its preparation step in advance, followed by calcining the resultingmixture at a high temperature at which the vapor pressure of theoxide(s) of at least one of Mo, W and Sn is elevated, may also beemployed. Still further, the above-mentioned catalyst may be impregnatedwith the compound(s) of at least one of Mo, W and Sn, followed bytreating the resulting material at a high temperature as describedabove. As to the treating temperature in this case, for example, a rangeof 450° to 650° C. particularly 500° to 600° C. is preferred in the caseof the compound of Mo, and a range of 500° to 750° C. is preferred inthe case of the compound of W, in order to obtain an adequate vaporpressure of the oxide thereof.

When the content of at least one of Mo, W and Sn is in the range of 0.2to 2.0% in terms of the percentage coating assuming that these metalcomponents are adsorbed on the catalyst to form a monomolecular layer,i.e. in the range of 2×10⁻⁶ mol/m² to 20×10⁻⁶ mol/m² in terms of the molnumber per the specific surface area, then the effectiveness ofpreventing the catalyst deterioration due to heavy metal vapors and thedenitration activity are both enhanced. If the above-mentioned molnumber is less than 2×10⁻⁶, the catalyst performance is notably reduced,while if it is larger than 20×10⁻⁶, the initial catalyst performance isinferior.

The preferred range of the mol number per the specific surface area ofthe catalyst is 5×10⁻⁶ to 15×10⁻⁶ mol/m².

The catalyst of the present invention, after subjected to theabove-mentioned treatment, may be molded into a predetermined shape, forexample a honeycomb or a tablet, or coated onto a metal plate to form acatalyst plate. The above-mentioned treatment may be applied aftermolding of the catalyst. Further, when the catalyst is molded or formed,various auxiliaries, reinforcing agents such as inorganic fibers orbinders may be added to the catalyst.

According to the present invention, it is possible to subject exhaustgases, for example those from a combustion apparatus like a boilerhaving an ash-circulating line as shown in FIG. 2, to denitrationtreatment, which has been difficult to realize for conventionalcatalysts due to notable reduction in the activity, with a simplicityand using the same amount of catalyst as that of conventionaldenitration.

In the production process of the catalyst for achieving the secondobject of the present invention, an active element of V, etc. isadsorbed onto metatitanic acid in the first step and then calcined,whereby it is highly dispersed on TiO₂ to form highly active sites.Further, since dehydration and sintering at low temperatures arecompleted by the calcination, occurrence of cracks and reduction in thestrength due to shrinkage of the resulting molded products by thecalcination are prevented.

By wet-kneading the catalyst component and MoO₃ at the second step, aportion of the catalyst component forms contact points of TiO₂ --TiO₂particles and another portion thereof forms a state where it isphysically coated by MoO₃.

Further, by the high temperature calcination at the third step, MoO₃ isdispersed on the titanium oxide catalyst surface. Thereby, theresistance to poison of the catalyst is highly improved.

As described above, according to the process of the present invention, araw material is prepared in advance under conditions capable ofincreasing the catalyst activity and the strength, in the first step,and the resulting material is kneaded at the second step, wherebycontact points of TiO₂ --TiO₂ particles indispensable for improving thestrength are formed to thereby obtain a catalyst having a very highstrength.

Further, MoO₃ is still in a physically mixed state at the time ofmolding, but it coats the catalyst surface for the first time at thethird step. In the conventional production process, when Mo raw materialis added into a metatitanic acid slurry, MoO₃ coats the catalystparticle surface to hinder contact of TiO₂ --TiO₂ particles and therebyreduce the catalyst strength. In the present invention, such a problemdoes not occur.

Furthermore, since MoO₃ has been physically uniformly mixed at thesecond step, it is possible to obtain a uniform and superior moldedproduct of the catalyst as compared with that obtained according to aprocess of treating a once molded product of catalyst with MoO₃ vapor toenhance to its resistance to poison.

Still further, if water-insoluble MoO₃ is not used in the presentprocess, but a water-soluble molybdenum compound such as ammoniummolybdate is used, the Mo compound intrudes into contact points of TiO₂--TiO₂ formed at the first step to make it impossible to obtain acatalyst having a high strength. Whereas, when water-insolublemolybdenum oxide is used, no breakage of contact points of TiO₂ --TiO₂occurs; hence a catalyst having an extremely high strength is obtained.

The present invention will be described in more detail by way ofExamples.

EXAMPLE 1

Ammonium metavanadate (NH₄ VO₃)(25.8 g) was added to a slurry (1 kg) ofmetatitanic acid having a titanium oxide contant of 30% by weight,followed by kneading the mixture on heating by means of a kneader whilevaporizing water to obtain a catalyst paste, thereafter drying thispaste at 180° C. for 12 hours, grinding the dried material by means of ahammer mill, calcining the resulting material at 550° C. for 2 hours,press-molding the resulting powder into a material of 13 mm in diameterand 5 mm in length, grinding this material to a particle size of 10 to20 meshes to obtain Ti/V catalyst, introducing this granular catalyst(50 g) together with molybdenum trioxide (MoO₃) (50 g) into a crucibleequipped with a lid, keeping the contents at 550° C. for 3 hours tosubject the catalyst to deposition treatment with MoO₃ vapor, coolingthe resulting material down to room temperature and taking out thecatalyst granules alone. The quantity of MoO₃ adsorbed onto the catalystwas 4.3% by weight as a result of chemical analysis.

COMPARATIVE EXAMPLE 1

Example 1 was repeated except that MoO₃ was not introduced into thecrucible.

EXAMPLES 2 to 6

A Ti/V catalyst powder was first obtained in the same manner as inExample 1, followed by mixing therewith MoO₃ powder in proportions of 2,5, 10, 15 and 20% by weight by means of a dry mixer, press-molding theresulting respective powders into a material of of 13 mm in diameter and5 mm in length, grinding it to a particle size of 10 to 20 meshes,introducing the resulting granules into a crucible equipped with a lidand heat-treating the contents at 550° C. for 3 hours.

COMPARATIVE EXAMPLES 2 to 6

Examples 2 to 6 were repeated except that the heat-treating temperaturewas changed to 400° C.

EXAMPLES 7 to 9

In Example 4, the same mol of copper nitrate, iron nitrate or manganesenitrate was added in place of ammonium metavanadate, followed by thesame process as in Example 4 to obtain Ti/Cu, Ti/Fe and Ti/Mn catalysts.

COMPARATIVE EXAMPLES 7 to 9

Examples 7 to 9 were repeated except that no MoO₃ deposition was carriedout to obtain catalysts.

EXAMPLE 10

In place of MoO₃ deposition in Example 1, the catalyst was impregnatedwith a 30% by weight hydrogen peroxide aqueous solution of ammoniumparatungstate so as to give the same mol as that of MoO₃, followed bydrying the resulting material at 180° C. for 2 hours and then calciningat 700° C. for 2 hours.

EXAMPLES 11 to 14

To a metatitanic acid slurry (50 kg) having a TiO₂ content of 30% byweight were added ammonium metavanadate (0 kg, 0.25 kg,0.50 kg or 1.02kg) and ammonium molybdate (3.68 kg, 3.72 kg, 3.76 kg or 3.81 kg),followed by kneading the mixture on heating by means of a kneader toobtain a paste having a water content of 34%, molding the paste into arod shape of 1 mm in diameter by means of an extrusion-granulator,drying it down to a water content of 3% by means of a fluidized beddryer, grinding the resulting material to a size of 90% or more in termsof 100 mesh pass by means of a hammer mill, preliminarily calcining theresulting powder at 350° C., adding water and kaolin inorganic fibers in15% by weight to obtain a paste, pressure-applying the paste by means ofrolls onto a metal substrate obtained by subjecting a thin plate of SUS304 of 0.3 mm thick to metal lath processing and then flame-sprayingmolten aluminium onto the resulting material, air-drying the resultingmaterial for 12 hours and calcining at 550° C. for 2 hours to obtaincatalysts.

COMPARATIVE EXAMPLE 10

Example 14 was repeated except that no molybdenum was added, to obtain acatalyst.

EXAMPLE 15

In order to enhance the catalyst strength of the catalyst of Example 14,the catalyst was impregnated with an aqueous solution of aluminumsulfate (concentration: 350 g/l), followed by drying and thencalcination at 500° C. for 2 hours.

The respective catalysts of the above-mentioned Examples and Comparativeexamples were subjected to a duration life test under conditionsindicated in Table 1. The conditions are those which simulatedenitration conditions directed to coal combustion exhaust gases havingarsenic suboxide generally known as one of volatile heavy metals incoal.

                  TABLE 1                                                         ______________________________________                                                     Examples 1˜10                                                                        Examples 11˜15                                Item         Comp. exs. 1˜9                                                                       Comp. ex. 10                                        ______________________________________                                        Catalyst     10˜20 mesh                                                                           20W × 100L - 1t                               shape                                                                         Quantity of  4 ml         3 sheets                                            catalyst                                                                      Gas                                                                           composition                                                                   NO           200 ppm                                                          NH.sub.3     240 ppm                                                          SO.sub.2     500 ppm                                                          CO.sub.2     12%                                                              O.sub.2       3%                                                              H.sub.2 O    12%                                                              As.sub.2 O.sub.3                                                                            14 ppm                                                          N.sub.2      remainder                                                        Gas quantity 240 l/h      180 l/h                                             Temperature  350° C.                                                                             350° C.                                      Test time    20 h         20 h                                                ______________________________________                                    

Next, Table 2 shows the results of life test, the specific surface areasand the contents of sulfate group in the catalysts of Example 1 andcomparative example 1.

                  TABLE 2                                                         ______________________________________                                               Percentage   Content   Specific                                               denitration (%)                                                                            (wt %)    surface area                                    Catalyst Initial After 20 hrs.                                                                            Mo   SO.sub.4                                                                           (m'/g)                                  ______________________________________                                        Example 1                                                                              98      92         4.3  0.3  60                                      Comp. ex. 1                                                                            97      33         0    1.7  67                                      ______________________________________                                    

It is seen from the results of Table 2 that MoO₃ adsorption treatmentmakes it possible to remarkably reduce the rate of deterioration ofdenitration catalyst. Further, in view of the fact that the quantity ofSO₄ in the catalyst was reduced by MoO₃ adsorption, it is presumed thatSO₄ might have been replaced by MoO₃.

FIG. 1 shows a chart obtained by plotting the denitration performancesafter the life tests of Examples 2 to 6 and Comparative examples 1 to 6,relative to the mol numbers of MoO₃ per the catalyst surface area. It isseen from the results of FIG. 1 that the catalysts of the Examplesobtained by adding MoO₃ and then treating the resulting material at ahigh temperature of 550° C. have a high denitration activity even afterthe life test and also a high durability. Further it is also seen thatthe quantity thereof added is effective within 2×10⁻⁶ to 20×10⁻⁶ mol/m²,particularly 5×10⁻⁶ to 15×10⁻⁶ mol/m² per the specific surface area.

Table 3 shows the results of life test of the catalysts of Examples 7 to10 and Comparative examples 1 and 7 to 9.

                  TABLE 3                                                         ______________________________________                                                              Percentage                                                                    denitration (%)                                                                             After                                     Catalyst  Composition (atomic ratio)                                                                      Initial 20 hrs.                                   ______________________________________                                        Example 7 Ti/Cu/Mo = 85/5/10                                                                              93      72                                        Example 8 Ti/Fe/Mo = 85/5/10                                                                              95      77                                        Example 9 Ti/Mn/Mo = 85/5/10                                                                              83      69                                        Example 10                                                                              Ti/V/W = 86/4/10  95      77                                        Comp. ex 7                                                                              Ti/Cu = 85/5      93      14                                        Comp. ex 8                                                                              Ti/Fe = 85/5      96      28                                        Comp. ex 9                                                                              Ti/Mn = 85/5      88      40                                        Comp. ex 1                                                                              Ti/V = 86/4       98      33                                        ______________________________________                                    

It is seen from the results of Table 3 that Cu, Fe and Mn other than Vare also effective as the active component in the catalyst of thepresent invention.

Further, Table 4 shows the test results of Examples 11 to 15 andComparative example 10, and this Table also shows that the catalyst ofthe pesent invention has a superior durability.

                  TABLE 4                                                         ______________________________________                                                              Percentage                                                                    denitration (%)                                                                             After                                     Catalyst  Composition (atomic ratio)                                                                      Initial 20 hrs.                                   ______________________________________                                        Example 11                                                                              Ti/Mo = 90/10     92      68                                        Example 12                                                                              Ti/V/Mo = 89/1/10 92      88                                        Example 13                                                                              Ti/V/Mo = 88/2/10 95      92                                        Example 14                                                                              Ti/V/Mo = 86/4/10 98      94                                        Comp. ex 10                                                                             Ti/V = 86/4       98      54                                        ______________________________________                                    

Further, with the catalyst of Example 14, steel sand of 10 to 20 meshes(8 kg) was dropped onto the respective surfaces of 100 test pieces ofthe catalyst tilted by 45° from the height of 1 m to observe theabrasion at that time and thereby evaluate their strengths. As a result,it was seen that only about 1/8 of the abrasion in the case of Example13 wherein the catalyst was not impregnated with aluminum sulfateoccurred. It was confirmed that even when the catalyst was combined witha strength-improving agent as described above, deterioration of thecatalyst of the present invention due to heavy metals did not increaseas shown in Table 4.

The invention by which the above-mentioned second object of the presentinvention is achieved may be concretely carried out according to aprocess as described below.

In the preparation of the catalyst raw material as the above-mentionedfirst step, hydrated titaniums such as metatitanic acid (TiO(OH)₂),orthotitanic acid (Ti(OH)₄), etc. may be used as the titanium oxide rawmaterial. Practically, a slurry of metatitanic acid according tosulfuric acid process may be used. Further, V, Cu, Fe and Mn as activecomponents may be in any compound from, but oxo acid salts of ammoniummetavanadate (NH₄ VO₃), etc. and besides, nitrates, sulfates, etc.thereof afford good results. The quantity of these catalyst rawmaterials added to TiO₂ is preferred to be 0 to 20%, and in of catalystactivity and activity for SO₂ oxidation reaction as a side reaction, itis preferred to be 0.1 to 5% by atom. As to the means for mixing theboth, usually a means of kneading the both on heating in the presence ofwater is preferably employed, but any means may be employed as far asthe both can be uniformly mixed. The resulting pasty substance obtainedby the mixing is molded according to extrusion-granulation process,followed by drying, calcining at 300° to 600° C. and if necessary,grinding.

MoO₃ used at the second step is added in 3 to 20% by weight to thecatalyst raw material and the mixture is wet-kneaded by means of akneader or the like. In this case, besides such mixing, it is preferredto vigorously mix them in a state of a low water content, in order tophysically coat the catalyst particle surface with MoO₃. As far as thisobject is achieved, of course any means may be employed. Further, whenfibrous materials such as ceramics wool are added and the mixture iskneaded, the catalyst strength is further improved, and when asurfactant is added, dispersibility is improved to thereby improvemoldability; hence good results are obtained. As to a fabrication atthis step, any process may be employed such as a process of applying thepaste onto a metal substrate by means of roll to prepare a platecatalyst, a process of subjecting the paste to extrusion molding toprepare an honeycomb like catalyst, etc.

Further, in advance of the calcination at the third step, it ispreferred to air-dry the fabricated material of catalyst, followed bydrying on heating. As to the calcination conditions, calcination iscarried out usually at 400° to 600° C. in the atmosphere. If thetemperature is low, dispersing of MoO₃ is insufficient, while if it ishigh, recrystallization of thermally dispersed MoO₃ occurs. It ispreferred to choose a range of 450° to 550° C. Since the vapor pressureof MoO₃ is high at such calcination temperatures, scattering of MoO₃occurs somewhat, but if the calcination is carried out in a closedvessel, it is possible to prevent such scattering to thereby improve thecatalyst properties.

The above steps will be described in more detail by way of the followingExamples.

EXAMPLE 16

Ammonium metavanadate (NH₄ VO₃)(0.41 kg) was added to a slurry (20 kg)containing 30% by weight of meta-titanic acid as TiO₂ and 2.7% by weightof sulfuric acid, followed by kneading the mixture on heating by meansof a kneader to obtain a paste having a water content of 38%,granulating this paste into a granular shape of 3 mm φ, drying thegranules while feeding hot air at 150° C., preliminarily calcining theresulting granular catalyst in the atmosphere at 500° C. for 2 hours,and further grinding it into fine powder of 100 meshes or smaller bymeans of a hammer mill.

To this powder were added molybdenum trioxide (MoO₃, industrial gradereagent) (0.63 kg) and water (3.0 kg), followed by kneading the mixtureby means of a kneader for 30 minutes, thereafter adding silica-aluminafibers (1.0 kg) and further kneading the mixture for one hour to preparea paste for molding.

This paste was applied onto a metal lath of SUS 304 of 0.8 mm thick and500 mm wide subjected to flame-spraying with metal aluminium, by meansof a roll press so as to embed the holes of the lath, followed byair-drying it in air at 550° C. for 12 hours and calcining the resultingcatalyst molded product for 2 hours. The composition of the resultingcatalyst was Ti/Mo/V=91/5/4 in an atomic ratio.

EXAMPLES 17 to 19

The ratio of Ti/Mo/V in Example 16 was replaced by 94/2/4, 86/10/4 or76/20/4 to prepare similar catalysts.

COMPARATIVE EXAMPLE 11

Addition of MoO₃ in Example 1 was not carried out to prepare a catalyst.

COMPARATIVE EXAMPLE 12

MoO₃ in Example 16 was replaced by ammonium molybdate (3(NH₄)₂O.7MoO₃.4H₂ O)(0.77 kg) and this was directly added to a slurry ofmetatitanic acid, to prepare a catalyst.

EXAMPLES 20 and 21

Example 16 was repeated except that ammonium metavanadate was replacedby iron nitrate (Fe(NO₃)₃. 9H₂ O)(1.41 kg) or manganese nitrate(Mn(NO₃)₂.6H₂ O) (1.00 kg), to prepare catalysts.

COMPARATIVE EXAMPLES 13 and 14

Addition of MoO₃ in Examples 20 and 21 was not carried out to preparecatalysts.

COMPARATIVE EXAMPLE 15

Example 16 was repeated except that MoO₃ was replaced by water-solubleammonium molybdate (0.77 kg) to prepare a catalyst.

With the catalysts obtained in the above Examples and Comparativeexamples, their denitration performances and abrasion resistances wereexamined. The results are shown in Table 5. The denitration performancesreferred to herein mean those at the intial period and after thedeterioration test in the case where a forced deterioration test wascarried out for 50 hours under conditions of Table 6 wherein As₂ O₃vapor was contained in 1.4 ppm. Further, the percentage abrasion refersto the percentage reduction in the catalyst weight in the case where thecatalyst was exposed into an air current (flow rate: 12 m/sec)containing a coal combustion ash (350 g/Nm²) for 100 hours. As shown inTable 5, any of the catalysts of the present invention have a lowpercentage abrasion and also exhibit a high percentage denitration afterdeterioration. Whereas, in the cases of Comparative examples 11, 13 and14 wherein no MoO₃ is contained, activity reduction at the time ofdeterioration test is notable; hence this evidences that addition ofMoO₃ of the present invention is effective for preventing thedeterioration. Further, Example 16 and Comparative example 12 are samein the composition, and nevertheless, the percentage abrasion of theformer is low; this evidences that the preparation process of thepresent invention is a superior process in the aspect of improving theabrasion strength. It is seen from the results of Examples 16 to 19having varied the contents of Ti and MoO₃ that when the content of MoO₃is low, there is a tendency that reduction in the percentage denitrationaccompanying the deterioration increases, while when the content is toohigh, there is a tendency that the activity and the strength both lower.In comparison of the Examples with Comparative examples, it is seen thatMoO₃ is effective in 2 to 20% by atom, and particularly superiorspecific features are exhibited in 2 to 10% by atom.

FIG. 2 shows results obtained when the catalysts obtained in Example 16where the preliminary calcination temperature was varied were subjectedto the above abrasion test. It is apparent from this FIG. 2 that thepercentage abrasion is low at a preliminary calcination temperature of300° to 600° C. and catalyst having a high strength is obtained. Thus itis indispensable for improving the abrasion strength to add MoO₃ afterthe catalyst has been subjected to preliminary calcination in advance.

Further, FIG. 3 shows the percentages denitration before and after thecatalyst deterioration test in the case where the catalyst calcinationtemperature in Example 16 was varied. As apparent from this FIG. 3,calcination of catalyst after addition of MoO₃ has a function ofreducing the catalyst deterioration. As seen in the X-ray diffractionpattern of the catalyst of Example 16, a peak attributing to MoO₃ is notobserved after calcination; hence the above function is an effectaccompanying the thermal dispersing of MoO₃.

Further, when the results of Example 16 are compared with those ofComparative example 15 in Table 5, it is seen that in the case ofComparative example 15 using ammonium molybdate as molybdenum rawmaterial, the percentage abrasion of catalyst notably increased. Theprocess of using water-insoluble MoO₃, bringing it into a physicallymixed state and then thermally dispersing it, as in the presentinvention, displays a notable effect of improving the strength.

As described above, the present invention is achieved by adding MoO₃ toa titanium oxide catalyst prepared in advance and preliminarilycalcining the mixture, followed by molding and calcining the mixture.

EXAMPLE 22

The catalyst-molding process in Example 16 and Comparative example 12was replaced by an extrusion-molding process by means of a moldingmachine provided with a die having a hole diameter of 10 mm φ to obtaina columnar catalyst of 10 φ.

The crushing strength of the resulting catalysts in the radial directionthereof was measured by Kiya type tablet strength meter. As a result,the strength of the former catalyst was 13 kg, whereas that of thelatter was 5.4 kg. Further, the surface of the molded product of thelatter caused fine split, whereas the latter did not cause such aphenomenon. This is considered due to the effect of MoO₃ upon improvingthe fluidability of paste. Thus the process of the present invention isalso suitable to extrusion-molding into honeycomb, cylindrical orcolumnar catalyst.

According to the present invention, since it is possible to retain ahigh denitration performance for a long time even in exhaust gasescontaining vapors of heavy metal compounds, it is possible to realize adenitration apparatus by the use of which the quantity of catalyst usedis small. Particularly in the case of an ash-circulating boiler, sincethe concentration of heavy metal vapors is very high, the quantity ofconventional catalysts used is calculated to be twice to three times thequantity in the case of conventional denitration apparatus, whereas inthe case of the catalyst of the present invention, there are an effectthat it is almost unnecessary to increase the quantity of the catalystused, and the like effect. Further, according to the present invention,catalyst deterioration due to catalyst poisons in exhaust gases is sosmall that it is possible to realize an exhaust gas denitration catalystwhich is small in the catalyst abrasion due to combustion ash. Further,the process of the present invention is a process having enhanced thestrength and resistance to poison of catalyst according to a preparationbased on the specific features of the catalyst components so that it ispossible to obtain a catalyst having a superior performance through farsimpler steps than those of binder addition process or the like.

                                      TABLE 5                                     __________________________________________________________________________                      Percentage                                                                    denitration                                                                         After  Percentage                                            Composition      deterioration                                                                        abrasion                                       Catalyst                                                                             (atomic ratio)                                                                           Initial                                                                             test   (%)                                            __________________________________________________________________________    Example                                                                            16                                                                              Ti/Mo/V = 91/5/4                                                                         98 or more                                                                          96 or more                                                                           2.7                                                 17                                                                              Ti/Mo/V = 94/2/4                                                                         98    81     2.5                                                 18                                                                              Ti/Mo/V = 86/10/4                                                                        96    94     4.2                                                 19                                                                              Ti/Mo/V = 76/20/4                                                                        93    92     8.2                                                 20                                                                              Ti/Mo/Fe = 91/5/4                                                                        93    75     2.6                                                 21                                                                              Ti/Mo/Mn = 91/5/4                                                                        95    88     2.7                                            Comp. ex                                                                           11                                                                              Ti/V = 96/4                                                                              98    54     3.5                                                 12                                                                              Ti/Mo/V = 91/5/4                                                                         98    94     17.3                                                13                                                                              Ti/Fe = 96/4                                                                             90    40     3.0                                                 14                                                                              Ti/Mn = 96/4                                                                             92    47     3.1                                                 15                                                                              Ti/Mo/V = 91/5/4                                                                         98    94     10.7                                           __________________________________________________________________________

                  TABLE 6                                                         ______________________________________                                        Item            Conditions                                                    ______________________________________                                        Catalyst amount 20 mm × 100 mm - 3 sheets                               Gas amount      3 l/min                                                       Temperature     350° C.                                                Gas composition                                                               NO              200 ppm                                                       NH.sub.3        240 ppm                                                       SO.sub.2        500 ppm                                                       SO.sub.3         50 ppm                                                       As.sub.2 O.sub.3                                                                               14 ppm                                                       CO.sub.2         12%                                                          H.sub.2 O        12%                                                          O.sub.2          3%                                                           N.sub.2         remainder                                                     ______________________________________                                    

What we claim is:
 1. A process for producing a catalyst for denitratingby catalytic reduction using ammonia an exhaust gas discharged from anash-circulating boiler system in which heavy metal compounds areconcentrated, the process comprising the steps of:mixing and kneadingtitanic acid or titanium oxide with a compound of metal selected fromthe group consisting of V, Cu, Fe and Mn in a water medium to form aresulting paste; drying said resulting paste; calcining said resultingpaste to obtain a calcined material; grinding said calcined material toobtain a catalyst powder; and contacting said catalyst powder with vaporof a molybdenum compound, tungsten compound or tin compound to adsorbsaid compound on the catalyst powder.
 2. A process for producing acatalyst for denitrating by catalytic reduction using ammonia an exhaustgas, according to claim 1, wherein said step of contacting is carriedout so that said compound is adsorbed on said catalyst powder at adensity of 2×10⁻⁶ to 20×10⁻⁶ mol/m² of surface area of said catalystpowder.
 3. A process for producing a catalyst for denitrating bycatalytic reduction using ammonia an exhaust gas according to claim 1,wherein the tungsten compound is tungsten oxide.
 4. A process forproducing a catalyst for denitrating an exhaust gas according to claim1, wherein the tin compound is tin oxide.
 5. A process for producing acatalyst for denitrating an exhaust gas according to claim 1, whereinthe molybdenum compound is molybdenum oxide.
 6. A process for producinga catalyst for denitrating by catalytic reduction using ammonia anexhaust gas according to claim 1, further comprising the stepsof:applying a paste of said catalyst powder onto a substrate to obtain aresulting catalyst; and drying and calcining said resulting catalyst. 7.A process for producing a catalyst for denitrating by catalyticreduction using ammonia an exhaust gas discharged from anash-circulating boiler system in which heavy metal compounds areconcentrated, the process comprising the steps of:mixing and kneadingtitanic acid or titanium oxide with a compound of metal selected fromthe group consisting of V, Cu, Fe and Mn in a water medium to form aresulting paste; drying said resulting paste; calcining said resultingpaste to obtain a calcined material; grinding said calcined material toobtain a catalyst powder; mixing and kneading said catalyst powder witha molybdenum compound to coat physically the surface of the powder withsaid molybdenum compound; applying a paste of said catalyst powdercoated with said molybdenum compound onto a substrate to obtain acatalyst; and drying and calcining said catalyst.
 8. A process forproducing a catalyst for denitrating by catalytic reduction usingammonia an exhaust gas according to claim 7, wherein said catalystpowder is mixed and kneaded with an addition of fibrous materials.